Major problems exist in producing oil in heavy-oil reservoirs because of the high viscosity of the oil. Because of this high viscosity oil, a very high pressure gradient builds up around the wellbore, thereby utilizing almost two-thirds of the reservoir pressure in the immediate vicinity of the wellbore. Furthermore, as the heavy oils progress inwardly to the wellbore, gas in solution evolves more rapidly into the wellbore. Since the dissolved gas reduces the viscosity, this evolution further increases the viscosity of the oils in the immediate vicinity of the wellbore. Such viscosity effects, especially near the wellbore, greatly impede production, and the resulting wasteful use of reservoir pressure can reduce the overall primary recovery from such reservoirs.
Similarly, in light-oil deposits, dissolved paraffin in the oil tends to accumulate around the wellbore, particularly in the screens and perforations and within the deposit up to a few feet from the wellbore. This precipitation effect is caused by the evolution of gases and volatiles as the oil progresses into the vicinity of the wellbore, thereby decreasing the solubility of paraffin and causing it to precipitate. Also, the evolution of gases causes an auto-refrigeration effect which reduces the temperature, thereby decreasing the solubility of the paraffins. Similar to paraffin, other condensable constituents can also plug up, coagulate, or precipitate near the wellbore. These include gas hydrates, asphaltenes, and sulfur. In the case of certain gas wells, liquid distillates can accumulate in the immediate vicinity of the wellbore. Such accumulation reduces the relative permeability near the wellbore. In all such cases, such near wellbore accumulations reduce production rates and reduce ultimate primary recoveries.
Electrical resistance heating has been employed to heat the reservoir in the immediate vicinity of the wellbore. This has been the subject of recent pilot tests. Basic systems are described in Bridges U.S. Pat. No. 4,524,827 and in Bridges et al., U.S. Pat. No. 4,821,798. Such systems are applicable largely for new wells. Prior to installation, some modifications of casing near the wellbore are usually needed to permit electrical resistance heating in the reservoir near the wellbore. For a cased-hole completion, the electrode which is in the reservoir must be isolated from the casing by fiberglass tubing above and below the electrode as discussed in Bridges et al., U.S. Pat. No. 4,821,798.
In the case of open-hole completions, considerable modification of the downhole screen and near reservoir casing and tubing is required. For existing wells, the old gravel pack and screens must be removed and a new gravel pack and screen system installed so that an electrically isolated electrode can be positioned in the deposit. Such electrode may be part of the gravel pack and screening system.
Such near wellbore heating systems have been demonstrated to massively heat the reservoir just outside the wellbore and to reduce or eliminate many of the aforementioned thermally responsive flow impediments. Such elimination can result in demonstrated flow increases of 200 to 400%. These procedures are used primarily in new well installations for cased-hole completions, but can be also used for either new open-hole completions or to retrofit existing wells with open-hole completions.
However, open-hole modifications are largely limited to either new wells or existing wells that have a very high flow rate, because the cost of installing either a new well or repacking an existing open-hole completed well with a new electrode assembly and gravel pack system is large.
What is desired, then, is a method of retrofitting old wells, either cased or open-hole completions, which is inexpensive and yet heats some of the reservoir in the immediate vicinity of the wellbore adjacent to the formation as well as within the wellbore itself. One method of doing this has been attempted before with a mixed degree of success. This technique employs the use of cylindrical resistance heaters which are coaxially situated in the wellbore and are positioned in the wellbore immediately adjacent to the reservoir. The earliest patent in the literature on this subject matter was issued in July of 1865 in U.S. Pat. No. 48,584 which is described as an electric oil well heater. Since then, numerous patents have been issued which have covered this type of inside wellbore heating. Such past art includes Pershing U.S. Pat. No. 1,464,618, Stegemeier U.S. Pat. No. 2,932,352, McCarthy U.S. Pat. No. 3,114,417, Williams U.S. Pat. No. 3,207,220 and Van Egman et al., U.S. Pat. No. 4,704,514. Such systems, heating inside the wellbore, received considerable attention in the 1950""s and early 1960""s, with some improvements reported in some reservoirs and other reservoirs showing mixed results. One principal difficulty encountered with such heaters was that they burned out at intervals so frequent that their use could not be justified. Though some of the causes of the failure of these resistors were due to poor designs, some fundamental problems also exist which contributed to the burn-out problem.
The useful heat supplied by the cylindrical resistor flows out of the wellbore and into the formation by thermal conduction. At the same time, unavoidably, the flow of fluids inwardly into the wellbore removes, via convection, transfers heat transferred by convection from the formation toward the producing well. In the wellbore itself, the heat is further unavoidably removed from the annular space between the heater and the screen or casing, via convection caused by the upward flow of oil in the well. Therefore, in order to achieve a noticeable increase in temperature just outside of the wellbore, very high heater temperatures were required. Such higher heater temperatures may also be accompanied by the deposition of scale or products of low temperature pyrolysis on the heater. This further thermally isolates the heater, thereby causing requirements for even higher resistor temperatures, which further compounds the problem. As a consequence of this fundamental counter flow heat problem between outward thermal diffusion and inward thermal convection, such an approach would be effective only in slowly producing wells and would become decreasingly less effective as the flow rate was increased much above a few tens of barrels per day for typical installations.
One method to mitigate the aforementioned problem would be to create a situation such that the casing itself, in the completed zone, would provide the heat. Alternatively, for an open-hole completion, the screen and/or gravel pack might preferably provide the heat rather than a small diameter cylindrical resistor element coaxially located within the wellbore next to the producing zone. By so doing, the radius of the heat producing element or resistor could be extended from approximately 1 in. to about 8 in., depending on the diameter of the wellbore or screen in the completed zone. Such an arrangement would give at least a fourfold improvement in the amount of heat which could be transferred based on a given temperature of the heated element. In addition, such an arrangement would eliminate in the annulus convection heat losses in the annulus due to the upward thermal convection of the fluids once they entered into the wellbore itself.
Earlier techniques have been ineffectively addressed in two U.S. patents; 1) by A. W. Marr in U.S. Pat. No. 4,319,632 and 2) by S. D. Sprong in U.S. Pat. No. 2,472,445. In either case, no system is adequately described which embodies the use of such casing heating systems and which is combined with an efficient downhole power delivery and control system. For example, in the case of Marr, the electrical heating system had one electrical contact with the casing at the surface and the other contact in the producing zone. As a consequence, current flowed from the bottom of the casing up along the entire surface, thereby heating the entire casing string and adjacent formations. Such a system is quite inefficient, especially if high temperatures are desired. In the case of Sprong, the system heated the casing by use of an induction eddy-current type heating applicator. However, the applicator as described had a large air gap between the applicator and the casing and, as a consequence, the reactive or inductive component was large, thereby creating a low power factor load on the power cable delivery system. Such low power factors result in inefficient delivery of power.
For aboveground equipment, any low power factor load which has modest power consumption (e.g., a few tens of kilowatts), and which is paired with high power factor higher power systems does not pose a problem. However, it is not readily recognized that delivering power over a half mile distance to a downhole load with a low power factor does represent a major power delivery problem and can result in cable overheating losses, cable breakdown, and other undesirable problems, especially if loads are in the order of tens of kilowatts or more. It also represents a less efficient method of power delivery.
Marr and Sprong do not address the issue of choosing operating parameters and the required additional subsystems or operation conditions that permit efficient power delivery. Such operating parameters include proper selection of the electrical waveform or frequency or proper locating and design of the casing wall heating tool. Additional subsystems (which may include a downhole matching network and control apparatus) are needed to prevent formation damage due to deposition of pyrolysis products of the incoming liquids in the immediate vicinity of the borehole and especially on the screens or perforations.
More recently, one patent has issued that remedies many of the difficulties with Matt and Sprong by Bridges, (Canadian Patent 2,090,629, issued Dec. 29, 1998) Electrical Heating System for Low-Cost Retrofitting of Oil Wells). This patent describes two generic casing heating systems, one that uses induction heating apparatus to heat the casing or screen by eddy-current effect and one that uses direct ohmic heating of the casing or screens. This latter approach uses a pair of contactors to supply heating current to a section of perforated casing or screen in the pay zone. To enhance power delivery efficiency, a downhole transformer is used to transform the very low impedance of the heated segment to a value much larger that the series impedance to the power delivery system.
Over the last few years, others* have developed and field tested an eddy-current current casing system very similar to that described by Bridges. (* Method and Apparatus for Subterranean Thermal Conditioning, Robert Isted, a published Canadian patent application No. 2208197, Electrical Induction Heating of Heavy Oil Deposits Using the Triflux System, by Homer Spencer, Nickles New Technology Magazine, Vol. 4, No. 2. June 1998 pp. 627-630, and Electrical Heating of Oil Wells Using the Triflux Method. Tesla Industries, 1998). Similar to that described by Bridges, the Isted/Spencer apparatus consists of a long, small-diameter, eddy-current heating coil that is positioned within the casing or screen that are within the pay zone. Each of these small diameter coils are stacked longitudinally on a single axis in groups of three, presumably to take advantage of a three phase 60 Hz power supply or to use existing three conductor armored cables. Each of the three coils is provided with a temperature sensor, but only one of the temperature sensors is used to control the heating. The three coils are packaged to withstand the bottom hole pressures. A downhole pressure sensor is also provided. A power conditioning unit is used to generate power in a suitable format under the control of the single downhole temperature sensor. Typical lengths of one or more groups of three coils are reported to range from 10 meters to 20 meters.
However, neither the Bridges or the Spencer/Isted apparatus or methods adequately account for the effects of heterogeneity found in typical deposits. While Spencer/Isted states xe2x80x9cThe principal control strategy is to maintain a constant temperature in the wellbore annulus in the vicinity of the inductors as measured by several temperature sensors deployed in the inductor assemblyxe2x80x9d but they do not provide the means to do so. For example they further state xe2x80x9c. . . the Triflux System heats quite evenly over the entire length and surface of the target interval.xe2x80x9d Additionally they note, xe2x80x9cThe main function of the PCU (Power Conditioning Unit) is to control the power input to the well by maintaining a constant temperature at one of the selected temperature sensors on the tool.xe2x80x9d
While not obvious, the above implementation of their strategy doesn""t lead to optimum operation. For example, consider a 3-meter pay zone that is to be heated by the above described casing/screen heating system. Past studies have shown that about 5 kW are needed to increase the temperature of one barrel of oil by 100xc2x0 F. For this example, we will only consider this energy to just raise the temperature of the oil, although additional energy will be expended over time to heat the formations very near the wellbore. Assume that over the length of the pay zone, a highly permeable 1-meter section exists near the bottom of the pay zone and that this zone will produce one barrel per hour by dissipating 5 kW per hour in the casing. This raises the temperature of both the casing and produced liquids by 100xc2x0 F. For simplicity assume that almost all of the production comes from this highly permeable region of the reservoir. However, to expend 5 kW within the casing near this highly permeable zone, an additional 10 kW will be expended in the upper 2-meters of the casing that is in low production zone. This occurs because the tool uniformly heats the casing throughout the pay zone. In this upper 2 meter section, the liquid that flows into the annulus from the reservoir is very small so that most of the 10 kW of heating will substantially increase the temperature of the liquids that are progressing upwards in the annulus of the casing from the permeable zone. One of two effects may take place: if a single, temperature-controlling sensor is near the top of the casing, the permeable zone will be under heated, and, therefore, only minimal stimulation benefits will occur. If a single, temperature-controlling sensor is located near the permeable zone, the upper part of 3 meter section will be overheated, and this excessive heating may cause premature failure of the eddy-current heating tool.
The above discussion neglects the energy that is lost to raise the temperature of the adjacent formations, especially where little or no liquid flows into the bore hole. This effect would temporally mitigate the excessive heating near the upper part of the bore hole when most of the production is from the lower section.
As opposed to the strategy of uniformly heating the casing across its entire span, a new strategy is needed to remedy the difficulties inherent with such uniform heating. A combination of several new criteria will be needed, especially after the initial warm up period:
(A) The spatial distribution of the temperature along the perforated casing should be uniform and not exceed a predetermined safe or economical value. The temperature should be limited so as to not degrade the heating tool or oil well completion components. Also, depending on the reservoir, operating at maximum safe operating temperature may not always result in the greatest cost benefit. As given in the preceding example, uniform heating (energy dissipation) along the casing heating tool will not generally achieve these goals.
(B) A practical alternative to (A), the spatial distribution of the temperature along the heating tool should be uniform and not exceed a safe or economical value.
(C) The spatial distribution of the heating (energy dissipation) along the perforated casing in the pay zone should be approximately proportional to the spatial distribution of the ingressing liquids along the perforated casing.
(D) The energy dissipation should also be proportional to the heat required to raise the temperature of a unit volume of produced liquids to a specified amount. For example, liquids with a high water content will require more energy than liquids with a very small amount of water.
(E) In reservoirs which have multiple producing zones that are separated by barren zones, the above criteria must be separately applied to each of the producing zones.
To realize the above criteria will require: (A) segmenting the casing heating functions of the tool into lengths that are smaller than the entire length of the perforated casing, (B) measuring the temperature near each of the segmented lengths, (C) controlling the dissipation of energy in each of the casing segments such that the maximum safe or economic temperature is not exceeded and (D) providing apparatus that permits control of the heating in terms of a specified preferred uniform or otherwise predetermined casing temperature profile.
Alternatively, the thermal heat transfer from or into the deposit near a segment can be calculated and used to simplify the design. Assuming that good reservoir data is available, the heat flows and temperatures near each segment can be calculated for a given thermal input. This calculation can be done by digital simulation programs that combine the electrical heating effects with reservoir analysis. One example of such a program is STARS that was evolved from a thesis by A. D. Herbert [entitled: xe2x80x9cNumerical simulation of electrical preheat and steam drive bitumen recovery process for the Athabasca oil sands, Department of Electrical Engineering, University of Alberta, 1986]. The reservoir portion of such programs considers the spatial distribution of the pore volumes in the reservoir, the oil saturation of the pore volumes, the viscosity of the oil, the relative permeability of the pore volumes, the reservoir pressure, gas saturation, over burden pressure, the thermal conductivity, the heat capacities and the convection of heat. The electrical portion considers the spatial distribution of the electrical conductivity and the power dissipation of electrical energy in the reservoir or in the casing. The resulting calculations include the spatial temperature distribution and production of fluids in response to the electrical heating. Also included are the heat transfers into and out of the formation.